At large airports, takeoff and landing slots, runway assignments, holding patterns, and approaches are handled by the tower's air traffic controllers during operating hours of the control tower. Smaller airports, however, rarely have the traffic to justify the expenses associated with the equipment and salaries of the tower crew. Instead, most small airports have their own Common Traffic Advisory Frequency (CTAF) which is unique for their geographic region. A CTAF is also usually assigned for after hours use at controlled airports. The CTAF of each airport is published and readily available.
Essentially, the CTAF serves as a radio bulletin board. A pilot approaching or departing an airport tunes to the CTAF channel for that airport and monitors for activity. The pilot will then broadcast his/her intentions and position so that other pilots also monitoring the CTAF channel can plan their own operations accordingly. Consequently, broadcasts on the CTAF are rarely to a specific listener. Instead, they are simply general declarations of an intended course of action to anyone currently using the airport. In this way, smaller airports have a system by which pilots can coordinate their activities without requiring a dedicated control tower.
In addition to the bulletin board function, a pilot can also ask for a radio check on the CTAF from other listeners and get information regarding the airport's services and other business functions. The limitations on CTAF usage are set forth by Federal Communications Commission Regulations.
On a related topic, many mid- and larger size airports have an Automatic Weather Observation System (AWOS) which, as its name suggests, automatically provides weather information over a dedicated frequency. Most commonly, the AWOS units are small weather stations that monitor wind speed, wind direction, dew point, and barometric pressure and then transmit this gathered information using a synthesized voice continually and repeatedly every one to three minutes. The frequency allocations for the AWOS are also published so that an incoming pilot, for example, can select an appropriate runway from the wind direction and speed transmitted by the AWOS system without requiring intervention from the ground.